Among switching power supply circuits is that which steps up a power supply voltage and outputs it, namely, a so-called step-up switching power supply circuit. FIG. 6 is a circuit diagram showing a typical step-up switching power supply circuit according to an earlier technology. As shown in this drawing, when a switching element SW1 formed from an N-type MOSFET is turned on in this switching power supply circuit, a coil current IL flows from a power supply 1 via a coil L and the switching element SW1, whereby energy is accumulated in the coil L. Then, the switching element SW1 is turned off, whereupon a counter electromotive force based on the energy accumulated during the on-period of the switching element SW1 is superposed on an input voltage VIN which is the output of the power supply 1. As a result, an output voltage VOUT stepped up to a predetermined voltage can be obtained at an output terminal OUT via a Schottky diode SBD. Here, a capacitor CL functions as a smoothing capacitor for an output current IOUT, while a capacitor CFB functions as a phase compensation capacitor.
On the other hand, the output voltage VOUT at the output terminal OUT is divided at a predetermined ratio by a resistor RFB1 and a resistor RFB2, and supplied to one input terminal of an error amplifier 2. The error amplifier 2 outputs an error signal Verr representing a deviation between the voltage divided from the output voltage VOUT and a predetermined reference voltage VREF supplied to the other input terminal. A PWM comparator 3 compares the error signal Verr with a ramp signal RAMP generated by a ramp generator 4 to form a PWM signal PWM, and outputs the PWM signal PWM. The switching element SW1 has its on-off action controlled by a switching control circuit 5 based on the PWM signal PWM. In this manner, the on-period of the switching element SW1 is controlled based on the duty of the PWM signal PWM, whereby the output voltage VOUT is adjusted to a predetermined value. A phase compensation circuit 6 performs the phase compensation of the error amplifier 2 in a high frequency band.
With such a step-up switching power supply circuit, even when the switching control of the switching element SW1 is stopped in terminating the stepping-up action of the step-up switching power supply circuit, the input voltage VIN is outputted, unchanged, to the output terminal OUT.
This problem is solved by using a switching element in place of the Schottky diode SBD, and cutting off the power supply side from the load side with the use of this switching element. If this switching element is formed from MOSFET, however, the problem arises that the power supply side and the load side are connected by a parasitic diode inherent in the MOSFET.
Patent Document JP-A-2006-304500 is present as a publicly known document which discloses a step-up switching power supply circuit solving the above-mentioned problem. The step-up switching power supply circuit disclosed in Patent Document JP-A-2006-304500 is composed of a first MOSFET which is a P-type switching element connected between the output side of a coil and an output terminal, and in addition to the first MOSFET, a second MOSFET as a P-type switching element which is connected between the output terminal and the back gate of the first MOSFET and whose gate is connected to an input terminal, and a third MOSFET as a P-type switching element which is connected between the input terminal and the back gate of the first MOSFET and whose gate is connected to the output terminal.
Because of the above-described configuration, not only a current path routed by the first MOSFET, but also a current path mediated by a parasitic diode incidental thereto can be shut down by the second MOSFET when a stepping-up action is stopped.